In sheet-fed offset printing machines, the print carrier can have a thickness of up to an order of magnitude of 1 mm and can be transported through the machine at speeds as high as 10,000 to 15,000 sheets per hour, causing the piles in the feeder of these machines to be depleted relatively quickly. In order to maintain a high speed print run without interruption, a new pile must be precisely positioned below the residual pile and then combined with the residual pile. For this purpose, automatic pile changing devices have been developed by which the residual pile can be removed from a pallet located on a main pile-carrying assembly, allowing the main pile-carrying assembly to be lowered so that a new pile can be placed on the main pile-carrying assembly in a position precisely below the residual pile. These known pile changing devices use horizontally movable bars (carrying bars) mounted on a vertically movable frame to receive the residual pile.
The major components of the main pile-carrying assembly of the feeder are a plate for holding the main pile of sheets and a lifting device for moving the plate up and down. The auxiliary pile-carrying assembly has a lifting device for moving a frame up and down, and the frame has a pair of horizontally moveable arms for holding a sheet pile. Both the main pile-carrying assembly and the auxiliary pile carrying assembly each have their own motors and controllers (drive units). The drive units, arranged upstream of the motors, can be used to select an operating point for each motor. This operating point represents the switch-on frequency and the switch-on period of the motors and is selected as a function of the operating cycle of the printing machine and the thickness of the print carrier. If the print carrier is thick and the operating cycle of the printing machine is high, the motors for the main pile-carrying assembly and the auxiliary pile-carrying assembly are consequently switched on more frequently and for longer periods of time. In order to keep the top of the feeder pile (held either by the main pile-carrying assembly or the auxiliary pile-carrying assembly) at the proper height for feeding to the printing machine, a pile height scanner is arranged at the top of the feeder. This pile height scanner detects the height of the feeder pile and sends corresponding signals to the respective drive units assigned to each motor.
During pile changing operations, the auxiliary pile-carrying assembly is moved vertically to a position that allows the carrying bars to be extended horizontally between the top of the pallet and the underside of the feeder pile. Insertion of the bars at the appropriate moment requires syncronicity between the lifting movement of the auxiliary pile-carrying assembly and the lifting movement of the main pile-carrying assembly. After the carrying bars of the auxiliary pile-carrying assembly have received the residual pile, the main pile-carrying assembly is then lowered with the pallet in order to load it with a new pile.
Since the auxiliary pile-carrying assembly and the main pile-carrying assembly each carry different loads, their associated motors and lifting devices have mechanical and electrical differences. In the lifting devices, for example, the corresponding traction mechanisms and transmissions are different. Thus, to achieve the required syncronicity between the auxiliary pile-carrying assembly and the main pile-carrying assembly, complex measures have to be taken.
One method of synchronization is to use a sensor to detect the movement of the main pile-carrying assembly and generate the appropriate movement commands for the motor of the auxiliary pile-carrying assembly. However, this complicates the construction considerably. Another possibility is to adjust the motor of the auxiliary pile-carrying assembly during the synchronous movement according to suitable standard algorithms based on signals from the pile height scanner. But this requires an additional control unit arranged upstream of the motor of the auxiliary pile-carrying assembly to evaluate the scanner signals or it would require the current control unit to have appropriate computing capacity.